1. Field of the Invention
The present invention relates to a resin composition for coating capable of forming a coating film which is excellent in formability, staining resistance and hardness, and has an external appearance excellent in gloss and vividness.
2. Description of the Prior Art
As resins contained in resin compositions for coating which have been used in recent years, there are alkyd resins, vinyl resins, and silicone resins. These resins are used for solvent type coatings, aqueous dispersion coatings and powdery coatings. However, coating films formed by using coatings containing these resins do not have satisfactory properties such as gloss and vividness of the external appearance, formability, staining resistance, and hardness.
Japanese Laid-Open Patent Publication Nos. 57-57746 and 63-108081 disclose resin compositions for coating capable of forming coating films which have excellent formability and high hardness; however, their formability and hardness are not sufficient. Japanese Laid-Open Patent Publication No. 53-2536 and Japanese Patent Publication No. 2-56375 disclose the introduction of a metal sulfonate group into a resin (polyester) for coating for the purpose of improving pigment dispersibility of the coating. A coating film formed by using a composition containing this type of resin for coating is excellent in gloss and vividness; however, the coating film is poor in density and staining resistance since the polyester contained in the composition is substantially soluble in organic solvents easily and has a low specific gravity of such as 1.265 or less. In the case where the glass transition temperature of the polyester is 40.degree. C. or more, a coating film obtained has relatively satisfactory hardness, however the toughness thereof is not sufficient since the specific gravity of the polyester is less than 1.265 (at 30.degree. C.), resulting in poor formability.
In general, in order to obtain a resin composition capable of forming a coating film which has satisfactory formability by using a solvent soluble polyester, it is required that the glass transition temperature of the polyester be 20.degree. C. or less. The coating film obtained by using this type of resin composition is soft and the hardness thereof is not satisfactory. On the other hand, a coating film with high hardness does not have sufficient formability. A coating film obtained by using this type of resin composition containing a solvent soluble polyester has an external appearance excellent in gloss, however the staining resistance thereof is poor. In contrast, a coating film obtained by a powdery coating using a resin composition containing a polyester with a specific gravity of 1.265 or more which is slightly soluble in organic solvents is excellent in formability and staining resistance and has high hardness. However, this type of composition is slightly soluble in organic solvents, so that the composition should be coated in a powdery state. As a result, the surface of the coating film obtained is rough, and the gloss and vividness thereof are poor. Moreover, in the case where the resin composition containing this type of polyester is coated in a powdery state, the, line speed is remarkably decreased compared with the case where the resin composition containing a solvent soluble type resin is coated. Furthermore, it takes much time to prepare a composition which can develop a desired color by using this type of polyester which is slightly soluble in organic solvents.